1. Field of the Invention
The present invention relates to a thin-film bulk acoustic oscillator incorporating a piezoelectric thin film and first and second electrodes disposed on both surfaces of the piezoelectric thin film, and to a method of manufacturing such a thin-film bulk acoustic oscillator.
2. Description of the Related Art
A reduction in size and an increase in usable frequency range have been sought for mobile telecommunications devices such as cellular phones that have been dramatically widespread. Accordingly, a reduction in size and an increase in usable frequency range have been desired for electronic components used in the mobile telecommunications devices, too.
Some mobile telecommunications devices comprise a duplexer for switching between a transmission signal path and a reception signal path, which allows a single antenna to be used for both transmission and reception. Such a duplexer comprises a transmission filter for allowing a transmission signal to pass therethrough and for interrupting a reception signal and a reception filter for allowing a reception signal to pass therethrough and for interrupting a transmission signal.
Surface acoustic wave filters have been recently used for the filters of some of the above-described duplexers. The surface acoustic wave filters have a feature that they are usable at frequencies up to 2 gigahertz and capable of being smaller in size compared to ceramic filters. However, if the future mobile telecommunications devices are designed to be used at frequencies of 2 gigaherz and higher, there are still many technical problems to be solved to make surface acoustic wave filters capable of being used at such high frequencies.
To solve these problems, attention has been given to devices called thin-film bulk acoustic oscillators. The thin-film bulk acoustic oscillator is the device utilizing bulk acoustic waves that are transmitted inside a piezoelectric thin-film in the direction of its thickness. A resonator incorporating the thin-film bulk acoustic oscillator is called a thin-film bulk acoustic resonator (that may be hereinafter referred to as FBAR), in particular. The FBAR allows variations in resonant frequency by changing the thickness of the piezoelectric thin film. It is estimated that the FBAR is capable of being used at frequencies of several gigaherz.
The characteristics of the thin-film bulk acoustic oscillator greatly depend on the thickness and quality of the piezoelectric thin film. A variety of methods have been thus proposed to improve the characteristics of the thin-film bulk acoustic oscillator. For example, the Published Unexamined Japanese Patent Application 2001-313535 discloses a technique for reducing the surface roughness of an electrode layer to be the base of the piezoelectric thin film. The Published Unexamined Japanese Patent Application 2002-372974 discloses a technique for reducing the surface roughness of the bottom surface of the piezoelectric thin film and thereby reducing the surface roughness of the top surface of the piezoelectric thin film.
One of the techniques relating to the surface acoustic wave elements is a technique for polishing the surface of a thin film formed to cover interdigital electrodes for reducing the occurrences of ripples, which is disclosed in the Published Unexamined Japanese Patent Application Heisei 1-233816 (1989).
The piezoelectric thin film of the thin-film bulk acoustic oscillator is made of zinc oxide (ZnO) or aluminum nitride (AlN), for example. The crystal of such a material may be column-shaped in some cases. To form the piezoelectric thin film of the thin-film bulk acoustic oscillator by using such a material, the piezoelectric thin film is formed by sputtering, for example, so that the crystal is formed into a column extending in the direction nearly vertical to the surface of the piezoelectric thin film to obtain an enhanced piezoelectric effect. The piezoelectric thin film thereby formed has an uneven top surface due to the columnar crystal especially when the thickness of the thin film is about 1 μm.
As mentioned above, the characteristics of the thin-film bulk acoustic oscillator greatly depend on the thickness of the piezoelectric thin film. In particular, the resonant frequency of the FBAR depends on the thickness of the piezoelectric thin film. Therefore, if the piezoelectric thin film has an uneven top surface as mentioned above, variations in thickness are created among portions of the piezoelectric thin film when seen microscopically. As a result, the characteristics of the thin-film bulk acoustic oscillator are reduced. The technique disclosed in the Published Unexamined Japanese Patent Application 2001-313535 is provided for reducing the surface roughness of the electrode layer to be the base of the piezoelectric thin film. The technique disclosed in the Published Unexamined Japanese Patent Application 2002-372974 is provided for reducing the surface roughness of the bottom surface of the piezoelectric thin film. However, it is difficult through either of these techniques to sufficiently reduce the projections and depressions of the top surface of the piezoelectric thin film that result from the column-shaped crystal of the material of the piezoelectric thin film. For example, the Published Unexamined Japanese Patent Application 2002-372974 discloses that the root mean square (RMS) roughness of the bottom surface of the piezoelectric thin film is 2.0 nanometers (nm) or smaller. However, when the crystal of the material of the piezoelectric thin film is columnar, it is difficult to control such that the root mean square roughness of the top surface of the piezoelectric thin film is 2.0 nm or smaller even if the root mean square roughness of the bottom surface of the piezoelectric thin film is controlled to be 2.0 nm or smaller.